Change ancestorStackingContextLayer to ancestorStackingNode.

Each time we used the ancestorStackingContextLayer we really wanted the stacking
node. This converts all usages to ancestorStackingNode.

BUG=302120

Review URL: https://codereview.chromium.org/26863004

git-svn-id: svn://svn.chromium.org/blink/trunk@160018 bbb929c8-8fbe-4397-9dbb-9b2b20218538